In the early stages of data handling, data generators, such as computers, generally stood alone. As indicated in FIG. 1, each such computer C.sub.1 and C.sub.N was, and still is, wired to a local ground where it is plugged into a source of power. A ground level lead GL travels throughout all of the computer logic and is at ground potential of a local ground. Peripheral equipment, such as terminals, printers, and the like, have their logic grounded in the same manner.
However, as technology advanced, a need developed to communicate between two such stations, such as between two computers, between one or more computers and a printer or display, or the like. A common means of connecting such units was, and still is, RS232 as indicated in FIG. 2. In RS232, one metallic conductor M.sub.1 takes data signals DS.sub.1 from unit T to unit R, and a second metallic conductor M.sub.2 takes data signals DS.sub.2 from unit R back to unit T. Still another metallic conductor G.sub.1 ties the ground G.sub.T of the unit T to the ground G.sub.R of the unit R.
Many units, such as units T and R shown in FIG. 2, include other control leads CL.sub.T and CL.sub.R for "handshaking". The handshaking terminal of one unit will "ask" a corresponding handshaking terminal of a second unit if that second unit is ready to receive data signals, indicate that the terminal is on, and then wait for a return signal to indicate readiness. Upon receipt of a ready signal, the first terminal transmits data signals to the second terminal. Similar kissoff signals and sequences are used at the completion of a data signal transmission.
This control procedure for separate lines is rarely used today. Most RS232 users insert the handshaking and kissoff control into the normal data stream by using special characters such as X.sub.ON /X.sub.OFF, or ACK (acknowledge), NAK (negative acknowledge), or the like. Such simple control characters can be passed along with the data at no additional expense or additional hardware.
While successful, RS232 still has several drawbacks. One such drawback is a limit of about fifty feet for an RS232 connection. If the connection exceeds fifty feet, there is a possibility of a degradation of speed and accuracy of data transfer. Furthermore, there may even be a possibility that hardware can be damaged. Still another problem which can arise if RS232 exceeds fifty feet is a possible increase in erratic operation. Often erratic operation problems manifest in errors that look like disc or CPU errors which are not associated with communications, so a user may waste time and money looking for an error in the wrong place.
However, with the advent of modern needs, RS232 connections are often pushed well past the fifty foot limit, and some connections have been pushed to over one thousand feet.
To increase accuracy, RS422 was developed. RS422 is shown in FIG. 3, and adds a mirror image signal DS.sub.1M and DS.sub.2M, in each direction. The transmitted data signal thus becomes a differential pair and can be received more accurately. In design concept, the receiving unit merely needs to look at the difference between the two lines M.sub.1 and M.sub.1M or M.sub.2 and M.sub.2M to determine if a "1" or a "0" has been transmitted.
While somewhat successful, even the RS422 still has drawbacks and still may generate errors, especially if used over great distances.
The inventor has discovered that the ground wire G used in RS232 and in RS422 has been a source of the errors commonly associated with these connections. As clearly seen in FIGS. 2 and 3, the ground lead G ties the grounds G.sub.T and G.sub.R together. However, the inventor has discovered that ground level potential associated with one unit, such as unit T, may be different from that ground level potential associated with another unit, such as unit R. This is especially the case if lightning strikes in the vicinity of the connection or of either unit. Switching of adjacent electrical equipment may also disturb a local ground.
Such ground level potential difference may even generate a voltage potential. However, any differences in ground potential are generally spread out evenly across the logic of a unit. However, if the grounds of the units are coupled together, this difference may be spread out across the logic of all of the units connected by the ground leads, such as ground lead G. The inventor has found that such ground coupling can lead to miscellaneous errors and even hardware damage.
Furthermore, data signals are generally transmitted and received by two tiny, generally inexpensive, chips called line drivers and receivers. The line receiver "receives" both the minute data voltage and the ground voltage. If the ground voltage is large, it may be larger than the data signal, and can thus cause errors in data transmission. Since a metallic ground path violates the integrity of a CPU, memory or disc, even if the unit is turned off, errors can occur. Still further, even in the case of the RS422, the ground wire coupling of units can still create the above-mentioned problems. The problems are exacerbated if voltage spikes or the like are induced in the ground lead.
The inventor has also found that even if elements such as modems, ground isolation elements and the like are used in each element, due to the existence of this ground coupling of the units, the problem still exists.
Yet another problem with the communication cable being connected to the ground leads of each unit is caused if one of the units is subject to voltage spikes or the like as above discussed. Such spikes may find their way onto the ground lead, and disturb any data that is being transmitted via a balanced line circuit. If the balanced line circuit uses the ground lead as a reference, the noise on the ground lead, even common mode type noise, may mask or disturb the small data signals if the circuit uses a differential input amplifier. While theoretically, common mode noise should be cancelled out in such a circuit, in practice, if the noise is high enough, it can saturate component amplifiers or distort their input. In such a case, noise may occur at the output. Any device using such ground and circuitry may be subject to errors, even in such common mode set up.
Still further, if there is any power applied to the connecting lead, any disturbance in such power may find its way to the equipment coupled to the connection lead.
Still a further drawback to many connection leads arises because they are not intended to be used beyond a short distance, and thus do not generally include any means for checking the integrity and polarity of the data signals being transmitted from one unit to another. Thus, a user of a unit that is located a great distance from a sending unit may receive data signals that he considers to be erroneous, and may not realize that the data signal was sent out incorrectly. This may lead to expensive checking of the system, and even downtime while a problem is being traced.
However, any communications system that requires great expense and effort to retrofit onto existing systems will not be able to achieve its fullest commercial success.
Therefore, there is a need for a data communications transceiver which prevents undesired coupling between data stations, especially via ground leads, yet which does so in an economic manner which is easily retrofit onto an existing system and which will permit high speed (i.e., 9600 baud) data transmission which is accurate and can occur over long distances (i.e., greater than fifty feet).